Some wireless telemetry systems (e.g., burglar alarms, fire alarms, power utility meters, leak detectors, environmental monitoring, temperature control, etc.) comprise many transmitters that periodically or sporadically transmit messages to one or more receivers. In these systems, each transmitter is located at a different place and transmits messages that indicate the status of sensors associated with the transmitter. A centrally located receiver receives messages from each transmitter.
Normally, the transmitters transmit messages that are as short as feasible and with the interval between the transmissions as long as feasible. This is advantageous for two reasons. First, it minimizes the average current drain in the transmitters, which are typically battery operated. Second, short and infrequent transmissions lower the probability that data is lost as a result of collisions that occur when two or more transmitters transmit at the same time.
However, if an urgency is detected by the sensor associated with the transmitter, the transmitter transmits immediately in order to notify the receiver of the urgency as soon as possible.
Typically, a telemetry system transmits at a single frequency, and thus is susceptible to narrowband interference and signal loss due to a phenomena known as “multipath fading.” As a consequence, the reliability of such systems is compromised or, conversely, the transmitted power has to be increased to overcome the fading, which results in larger power drain and shorter battery life. Furthermore, there usually are regulatory limits that restrict such transmitter power and thus limit the possible compensation for multipath fading by merely increasing the transmission power.
Because the multipath effect is highly sensitive to the frequency of the transmitted carrier, a system using multiple frequencies (e.g., a frequency hopping spread spectrum system, etc.) has the potential to eliminate these drawbacks. However, frequency hopping systems require a long acquisition time and are typically used in two way communication applications in which all the devices can be synchronized by continuously synchronizing with one master device or with each other using a variety of synchronization methods suitable for such case.
In other cases, to ease the synchronization problem, there are employed receivers that can simultaneously receive signals at many frequencies by making the receiver broadband or by using several receivers at the same time. Generally, these receivers suffer from performance degradation or high cost or both which makes them undesirable for low cost applications that require high reliability such as security systems.
One serious problem that must be addressed in battery operated systems concerns battery life, and, therefore, it is advantageous if a telemetry system could be devised that shortened a transmitted messages preamble. A short preamble, however, makes it difficult for the receiver and the transmitter to become and stay synchronized. This problem is exacerbated in some systems, such as security alarms, that require some messages to be conveyed to the system immediately without waiting for the scheduled transmission time.